The overall aim of this proposal is to determine the mechanism by which Cux1 regulates the expression of the cyclin kinase inhibitor p27 in kidney development and polycystic kidney disease. Cux1 is highly expressed in the nephrogenic zone during normal kidney development, but is sharply downregulated when nephrons undergo terminal differentiation. Cux1 is ectopically expressed in human ADPKD cells, and in mouse models of polycystic kidney disease, including Pkd1 null and cpk mice. Transgenic mice that ectopically express Cux1 develop renal hyperplasia resulting from the aberrant repression of p27. However, studies on the Cux1 transgenic mice demonstrate that ectopic expression of Cux1 alone is insufficient to cause cyst development. Rather, increased levels of Cux1 expression appear to be required for cyst progression. Our preliminary results suggest that reducing Cux1 levels in polycystic kidney disease results in increased expression of p27, leading to cell cycle arrest and decreased cyst growth. Based on these results, we hypothesize that the downregulation of p27 by Cux1 results in the proliferation defects observed in PKD. Thus, in specific aim 1, we will characterize cell proliferation in collecting duct cells isolated from mice carrying targeted deletions of both Pkd1 and Cux1, and determine whether Cux1 is required to form a repressor complex on the p27 promoter. Our preliminary results suggest that Cux1 interacts with Grg4 to recruit the histone deacetylases HDAC1 and HDAC3 to regulate p27 expression. Thus, in specific aim 2, we will define the interaction between Cux1, Grg4, and the p27 promoter, and use the chromatin conformation capture assay to characterize the repression complex formed on the p27 promoter. Finally, treatment of Pkd1 null embryos with HDAC inhibitors slows cyst progression. Thus, in specific aim 3, we will determine whether HDAC inhibition results in increased p27 gene expression, and whether Cux1 regulation of p27 in PKD is dependent on HDAC activity. These studies will provide novel insights into the mechanisms of cell proliferation during kidney development and polycystic kidney disease.